If then, the first promoters of Vaccination were led, from their ardour in a cause of such vital interest to the whole human race, and before time had been allowed to afford sufficient data whereon to found a more correct opinion, to conclude that Vaccination would, in every instance, prove an absolute and infallible preventive of future susceptibility of Small Pox infection, let us not, on that account, run into an error incalculably more dangerous in its tendency, and underrate or deny altogether the degree of security which it actually affords. That this is more than sufficient to render it worthy of universal adoption, there cannot be any reasonable doubt; but it would have been strange indeed, if, in the undeviating uniformity of its effects, Vaccination had formed a _single_ exception to the _law of uncertainty_ that attaches to every other agent, which it has, at any time, been permitted to mankind to employ for the promotion or preservation of the health of the human constitution: and, considering the innumerable varieties which prevail in the latter, we ought rather, perhaps, to feel surprise that the influence of Vaccination over all these varieties, should have been proved to be so similar as to have justified us in considering as a _general_, what was at first too hastily concluded to be an _absolute_ rule. That the great majority of instances in which Vaccination has afforded perfect security against any future attack of Small Pox, fully warrants this conclusion, the results already detailed are sufficient to prove. What proportion these cases bear to the comparatively few, wherein the protection has been less perfect, it will not be possible to discover, because we are unable to ascertain how many hundreds of thousands or millions have passed through the vaccinating process. But little difficulty would be encountered, however, in ascertaining the full amount of the few who have had any thing bearing the slightest resemblance to an attack of Small Pox after Vaccination: these, for the most part, have been carefully recorded, and a degree of importance has certainly been attached to them greater than is warranted, either by their number, their severity, or their results.